Leukocyte effects on the dimorphism of Coccidioides immitis

To assess the influences of leukocytes on the dimorphism of Coccidioides immitis, mycelial elements (arthroconidia) or spherules were incubated with polymorphonuclear leukocytes (PMNLs) or mononuclear leukocytes. In the presence of human leukocytes, arthroconidia became separated spherules as determined by electron microscopy but grew as mycelia when viable leukocytes were absent. Plasma was not required for this effect. In a new assay, PMNLs, lymphocytes, or monocytes from human blood as well as rabbit alveolar macrophages inhibited spherules from forming germ tubes. In flat-bottomed glassware, maximal inhibition occurred with PMNL:spherule ratios above 15:1, whereas in conical-bottomed glassware a ratio of 3:1 was maximally suppressive with PMNLs as well as mononuclear leukocytes. Cytochalasin B and colchicine blocked the effect of PMNLs but not that of other blood leukocytes on spherules. These studies indicate that leukocytes influence coccidioidal dimorphism toward the spherule phase. This effect may be important in host defenses against C. immitis.